The invention relates to an electronic trip device for a circuit breaker protecting an electrical power system comprising:
a current sensor per pole supplying a secondary current proportional to a current flowing in an associated conductor of the power system protected by the circuit breaker,
an electronic processing unit receiving the signals supplied by the sensors, and delivering a tripping order, with or without a delay, when the currents flowing in the conductors to be protected exceed preset tripping thresholds,
a power supply circuit supplying the electrical power to the electrical circuitry of the trip device and to a tripping relay, and comprising chopping regulation means.
State-of-the-art electronic trip devices comprise power supply circuits to supply electrical power to the electronic circuitry of the processing unit, and the energy required for operation of a tripping relay. In most trip devices, the electronic circuitry operating voltage and the tripping relay operating voltage are different. The electronic circuitry supply voltages are compatible with the operating voltages of the digital integrated circuits, about 5 V and accurately regulated. The energy required for operation of the relay must be compatible with the current delivered by the sensors and the opening voltage they can withstand without disturbing the current. This voltage is generally situated between 9 V and 25 V, and the trip device must be able to operate with a minimum current of 10 to 20 mA. As the relay operating current is greater than the minimum current supplied by the sensors, a decoupling capacitor enables the relay operating energy to be stored. The current from the sensors then charges the capacitor up to the relay operating voltage and, when a tripping order is applied to the relay, the capacitor discharges at high speed into the relay with a strong current generally greater than the current from the sensors. Regulation of this operating voltage is achieved in state-of-the-art manner by linear regulation which limits the voltage and absorbs the current from the sensors, or by chopping regulation. In chopping regulation the current from the sensors charges the storage capacitor, then, when the charging voltage exceeds a preset threshold, a regulating circuit orders turn-on of a transistor to branch the current off and stop the charging. The power supply chopping cycles, corresponding to turn-on and turn-off of the transistor, may be free or fixed with a variable duty cycle.
Linear regulation limits the operating voltage and absorbs the current from the sensors in this voltage. With this regulating mode, the power absorbed becomes very high when the current increases. Chopping regulation prevents excessive heating as the voltage present in the chopping means at the moment the current from the sensors flows is low. However, for low currents on sensor output, the voltage variations due to chopping disturb operation of the sensors and impair the shape of the current. These disturbances have an effect on the behaviour of the trip device since the currents from the sensors are used for the tripping functions.